Guardian: Petrolheads should look away now. Engineers and racing car enthusiasts are gathering in Germany today for a car race with a difference – one that does not reward the fastest car, but focuses instead on the most fuel-efficient. In this Shell sponsored Eco-marathon, the best cars could travel the entire length of Britain five times on a single gallon of petrol. More than 200 teams from 29 countries will battle for the EUR1,000 (£895) top prize in this annual green car rally, which is …